1. Field of the Invention
The present invention relates to a biodegradable biopolymer material, which is degraded while being decomposed by the action of an enzyme. The biodegradable biopolymer material is thus converted into small molecules. The present invention also relates to a method for the preparation of the biodegradable biopolymer material, as well as a functional material containing the material, such as a metal ion-adsorbing material, a sustained release carrier for a useful substance, a biological cell-growth substrate and a biodegradable water-absorbing material.
2. Description of the Related Art
Materials consisting of organic polymers and possessing biodegradability are well known in the marketplace. In the medical field, materials have frequently been used which are biologically decomposed and degraded through the action of an enzyme to form small molecules. Materials recently put into practical use include, as typical examples, poly(oxy-acids), such as poly(lactic acid) and poly(glycolic acid) among the biodegradable organic polymers. These materials have widely been used as implanting materials to be embedded in the living bodies, materials for in vivo delivery or carriers for sustained release of medicines.
Poly(lactic acid) and poly(glycolic acid) show excellent resistance to chemicals. Poly(lactic acid) and poly(glycolic acid) are non-toxic and susceptible to hydrolysis. Accordingly, they have widely been used as materials capable of being decomposed and absorbed in vivo. Moreover, poly(glycolic acid) can be prepared as a very high molecular weight polymer and therefore, it is useful as a material, which should have excellent mechanical or dynamic characteristic properties, such as high tensile strength. Poly(lactic acid) and poly(glycolic acid) have been used as, for instance, biodegradable and bioabsorbable suture.
Moreover, in the medical field, silk sutures from domesticated silkworm have been used for surgical operations. The use of the silk fiber from domesticated silkworm as sutures for surgical operations dates back to the beginning of the eleventh century. The total volume of the sutures traded in this country is equivalent to about six billion yen a year (in 1985), 46% of which corresponds to the volume of the silk sutures. The silk fiber is excellent in, for instance, tensile strength and knot strength and can easily be sterilized. For this reason, silk fiber has favorably been used for sutures. Even when judging from the actual conditions of the use of the conventional silk sutures, the silk fiber can be sterilized easily. It never biologically decomposes in a short period of time when embedded in the living body. Further, when it is implanted in the living body, it only causes an insignificant antigen-antibody reaction, if any, with the biological tissues.
The cocoon fiber (the silk fiber) is a protein fiber produced and spun by matured larvae of silkworm. The silkworms are divided into two groups, domesticated silkworms reared in farmhouses and wild type silkworms. Silk fibroin fibers are those obtained by removing sericin as an adhesive substance, which covers the surface of the cocoon fiber, by treating the cocoon fiber with, for instance, an alkali.
The silk fibers from wild silkworm in general mean those produced and spun by, for instance, Antheraea pernyi, Antheraea yamamai, Antheraea militta, Antheraea assama, Philosamia cynthia ricini and Philosamia cynthia pryeri. 
The foregoing silk suture is a non-absorbent material, which is never decomposed within a short period of time. Accordingly, it would remain in the living body after the suture. For this reason, it has been used for purposes different from those of the threads for a suture made from poly(oxy-acids), such as poly(lactic acid) and poly(glycolic acid), which are absorbed in the body and are decomposed into water and carbon dioxide within several weeks after the suture.
With respect to the foregoing metal ion-adsorbing material and sustained release carrier for useful substances consisting of the aforementioned biodegradable biopolymer, no product having satisfactory characteristic properties has been proposed yet.
As discussed above, poly(lactic acid) has been widely used as a biodegradable and bioabsorbable material. However, it suffers from the problems that the production cost thereof is too high, it is too expensive, it has high crystallizability, it is too hard, and it is inferior in the compatibility with soft tissues. Moreover, it also suffers from problems that its rate of decomposition cannot be easily controlled and that control of its biodegradability is also difficult, even if this material is chemically modified.
Further, fibrous poly(lactic acid) has a glass transition temperature similar to that of polyethylene terephthalate fiber. Accordingly, the poly(lactic acid) fibers possess mechanical properties similar to those observed for the polyethylene terephthalate fibers. However, poly(lactic acid) or the like has a crystallization velocity slower than that observed for polyethylene terephthalate and fibers of poly(lactic acid) are not sufficiently oriented or satisfactorily crystallized even when they are passed through the usual spinning and/or orientation steps. For this reason, additional problems arise when putting them into practical use. For instance, the tensile strength and dimensional stability of poly(lactic acid) are insufficient.
In addition, the higher the molecular weight of the foregoing poly(oxy-acids), the slower the rate of the decomposition thereof. Thus, it is necessary to produce poly(lactic acid) and poly(glycolic acid) whose molecular weight is controlled to control the decomposition speed of these polymers. However, the production of such polymers requires a lot of labor and the use of highly advanced techniques which require a great deal of skill. For this reason, the use of poly(oxy-acids) has been limited to medical applications, such as absorbent sutures and cosmetic applications. Accordingly, there has been a strong desire for a production process, which is inexpensive or economical, and does not require any skilled technique.
As discussed above, a suture of silk differs from sutures of poly(oxy-acids), such as poly(lactic acid) and poly(glycolic acid), which are decomposed into water and carbon dioxide in vivo. Accordingly, there is a strong desire for the development of a biodegradable material whose biodegradability in vivo can be controlled, which does not suffer from any problem concerning the biological safety, whose production cost is very low, which can biologically be decomposed without producing any cytotoxic products, which does not form any harmful substance such as formaldehyde as a by-product, and which is safe to the biological tissues.
The silk protein, which can be used as a raw material for the foregoing silk suture, is a naturally occurring polymer material produced through the biosynthesis of silkworms, which is excellent in the biological compatibility with the biological tissues and has good molding properties. Therefore, if by-products of silk, obtained in the process for preparing raw silk and silk products, are used as starting material for the sutures, one can save the cost of raw materials. Moreover, silk proteins include a large number of active sites rich in chemical reactivity. Therefore, the fields of application (such as the use as medical materials) can be widely extended if a technique, which permits the control of the biodegradability or biochemical properties of silk fibroin, can be developed, for instance, through hybrid processings and/or chemical modification. For this reason, there has been a strong desire for the development of a novel biodegradable material, which can be used effectively in the medical field, and which uses biopolymers from insects as starting materials and secondary substances capable of being combined (hybrid or hybridized with the former composite materials).